Ocean waves are generated by wind, and large waves are the result of storms that have already occurred far out at sea. Large waves can travel long distances without losing energy from internal viscous effects (friction). However, waves do get smaller as they spread out laterally over the sea.
Wave generation and propagation can be predicted accurately with modern computer models that are often coupled with weather models. So, one can know days in advance what the wave conditions will be near the shore (at least for large waves from storms that have already occurred).
Waves contain energy: potential energy due to the water surface being displaced from its calm position, and kinetic energy in the movement of the water particles. Wave energy is proportional to the square of the wave height. It travels with the wave, so that waves have a certain power, which is often called energy flux. Wave power is frequently specified as a density: a power per meter of crest width. Imagine you are looking at a wave that is moving towards you; the crest the is portion of the wave across its direction of travel. The power per crest width is the power in a 2D slice of the wave through its crest.
Check out the map of the US wave energy developed by the National Renewable Energy Lab (NREL). Click on Legend on the left and notice that the resource is specified in kW/m.
A renewable wave energy converter, often called a WEC, is a mechanical-electric generator. There are lots of different concepts: buoys that heave up and down, flexible rafts, flaps etc. However, they all have a two common features:
1) Wave energy is transformed into the mechanical energy of the movement of a body or multiple bodies by fluid pressure acting over the body surface.
2) Mechanical energy is transformed into another form via the damped relative motion between at least two bodies.
In some designs, there are intermediate steps, and in some cases the second body may be the earth, but the key idea is: the water pushes something that moves and that movement is converted into another form of energy.
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